William Parsons, 3rd earl of Rosse, also called (1807–41) Lord Oxmantown (born June 17, 1800, York, Eng.—died Oct. 31, 1867, Monkstown, County Cork, Ire.), Irish astronomer and builder of the largest reflecting telescope, the “Leviathan,” of the 19th century.

In 1821 Parsons was elected to the House of Commons. He resigned his seat in 1834 but in 1841 inherited his father’s title, becoming the 3rd earl of Rosse, and served as one of the Irish peers in the House of Lords.

Lord Rosse was obsessed with the idea of constructing a truly large telescope and worked for five years to find an alloy suitable for the mirror. His mirrors were made of speculum metal, an alloy of approximately two parts copper to one part tin by weight. (Some makers added traces of other metals.) Adding more copper makes the mirror less brittle and therefore less likely to break, but the mirror is more susceptible to the development of small surface fissures in the cooling process, tarnishes faster, and has a less white colour. Because he was at first unable to cast large pieces without using too much copper, his first 36-inch (91-cm) diameter mirror was composed of 16 thin plates soldered to a brass framework.

The moderate success of this telescope encouraged Lord Rosse to try to cast a solid 36-inch mirror. After much experimentation he succeeded in casting and cooling the mirror without cracking it, a serious problem in the construction of all large telescopic mirrors. In 1842 he began work on a mirror of 72-inch (183-cm) diameter. Three years later the four-ton disk was mounted, and the installation was completed at his Birr Castle estate in Ireland. Fifty-four feet (16 metres) in length, Lord Rosse’s telescope was used primarily to observe nebulae on those rare occasions when weather conditions permitted.

With his telescope, however, he discovered the remarkable spiral shape of many objects then classed as “nebulae,” which are now recognized as individual galaxies. His drawing of the spiral galaxy M51 is a classic work of mid-19th-century astronomy. He studied and named the Crab Nebula. He also made detailed observations of the Orion Nebula. Though his telescope was dismantled in 1908, it was not until the 100-inch (254-cm) reflector was installed in 1917 at the Mount Wilson Observatory in California that a larger telescope was used. The telescope was later reconstructed and the original masonry mounting restored; these can be seen in the castle grounds at Birr, Ire.